1. Field of the Invention
The present invention relates to liquid crystal display devices and in particular to liquid crystal display devices having reflectance enhancing thin films.
2. Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These conventional high density projection-type color display devices typically include a light source which emits white light. Dichroic mirrors separate the white light into its corresponding red, green and blue (RGB) bands of light. Each of these colored bands of light is then directed toward a corresponding liquid crystal light valve which in accordance with the image to be projected, either permits or prevents transmission of light there through. Those RGB bands of light which are permitted to be transmitted through the light valves are then combined by dichroic mirrors or a prism. A projection lens then magnifies and projects the image onto a projection screen.
The liquid crystal light valves used in the projection system include a matrix array of pixel electrodes coupled to corresponding thin film transistors. Driving circuits connected to the thin film transistors selectively switch the thin film transistors in the pixel array of the light valve on and off. A voltage applied to the pixel electrode via the thin-film transistor will vary the direction of orientation of the liquid crystal material on the pixel electrode and will modulate the light traveling though the liquid crystal. When no voltage is applied to the pixel electrode, the light transmitted through the light valve is either unchanged if the cells contain twisted nematic liquid crystal, or scattered if the cells contain polymer dispersed liquid crystal. Once a voltage is applied, the liquid crystal's optical characteristics are modified and the light traveling through the light valve cell is changed.
Conventional liquid crystal light valves are formed by confining a thin layer of liquid crystal material between a top plate and a bottom plate. The top plate is typically a glass substrate having one large electrode on a surface adjacent to the liquid crystal material. The bottom plate is typically a silicon substrate which couples to reflective electrodes on a surface adjacent to the confined liquid crystal material. These reflective electrodes define the plurality of above-mentioned pixels organized in a matrix array of rows and columns.
Projection systems using light valve devices are capable of high resolution and brightness. However, it is desirable to provide liquid crystal light valves which provide the brightest possible images with a high level of picture quality and a high level of picture reproducibility and which can tolerate high levels of light without reaching an operating temperature that requires extensive external cooling means to maintain an operating temperature that will not cause the liquid crystal material to fail. Furthermore, although the conventional projection system has many advantages such as being light weight and having a compact size, total light output (brightness), contrast ratio (amount of light reflected when the light valve pixel state is fully on divided, by amount of light reflected when the pixel state is off) and conversion of absorbed light to heat, remain significant impediments to high-quality picture display.
Picture definition is a general term for describing the general characteristic of crispness, sharpness, or image-detail visibility in a picture. Picture definition is dependent upon the contrast ratio, in that, the higher the contrast ratio the crisper the image. As a result, contrast ratio is an important characteristic of a picture-image medium.
To improve the contrast ratio, and the brightness of the image highly reflective metal has been provided on the surface of integrated circuit memory chips as explained in U.S. Pat. No. 5,497,025, incorporated herein by reference. In these conventional chips, a reflective metal reflector plate includes three reflective metal layers. The bottom layer of the reflector plate structure is formed of aluminum Al. The surface of this bottom layer is too rough a surface for reflection, so another metal layer formed of Ti (titanium) or TiN (titanium nitride), or the like, is deposited on the aluminum Al. The top layer of the metal reflector structure is formed of cold aluminum. This top metal layer is etched.
A passivation layer is added as a final layer. A two-part etch is performed to expose the metal reflector plate top surface. After conventional lithography and etching to define the area to be etched, a reactive ion etch is performed to reduce the passivation layer thickness in the region above the metal reflector plate. The second etch is a wet etch to remove the remainder of the passivation layer over the metal reflector plate.
The highly reflective plate disclosed in the above-referenced patent increases the reflectance of the metal surface to approximately 80%. This increase in reflectance corresponds to a contrast ratio of 40. When a highly reflective metal reflector plate is not patterned, the reflectance of the metal reflector may be increased to approximately 90%.
One problem with this highly reflective metal reflector plate, however, is that the processing of the aluminum metal produces less reflectance. The processes of etching the metal, stripping the photoresist, depositing the passivation layer, and etching the passivation away, all roughen the surface of the highly reflective metal reflector plate. This roughness creates low reflectance and consequently a reduction of the light available to create the projected image.
Thus, a need exists to improve the contrast ratio and the brightness of an image created with reflective mode displays. Fulfilling this need in turn allows the creation of a smaller pixel size. Consequently, the net benefit will allow for the production of high quality picture definition, and the minimization of the absorption of light projected onto the light valve by maximizing its reflectance.